Synthesis Of Amorphous Si-B-C-N Ceramics And BN Nanotubes Via Precursor Pyrolysis And Their Properties Study

Development of new energy sources and new materials is a powerful impetus to the progress of science and technology. Recently, Polymer precursor derived amorphous Si-B-C-N ceramics and BN nanotubes have received steadily growing interests as a result of their peculiar properties. Si-B-C-N ceramics possess an unusual set of properties at ultra high temperatures (1500-2000oC), including excellent creep and oxidation resistance. BN nanotube is a unique tubular material combining ultimate strength and stable dielectric properties,the measured Young's modulus of an individual BN nanotube exceeds that of all known semiconductor materials. Also, it has a constant band gap independent of the chirality of the nanotube which is totally different with C nanotube's. Both these two noval materials exhibit great potential applied value. Therefore, exploring their physical properties and developing correlative synthesis methods is of great significance not only to the fundamental research but also to the potential applications. In this paper, we synthesized amorphous Si-B-C-N ceramics and BN nanotubes by using the same polymer precursor and then sdudied their physical properties respectively. The main results are outlined as followings: 1.To study the electrical properties of amorphous Si-B-C-N ceramics, four samples were chosen: (a) SiCN annealed at 1100℃,(b)SiCN 5%B annealed at 1100℃,(c)SiCN 5%B annealed at 1400℃,(d)SiCN 10%B annealed at 1100℃. The dc-conductivity was measured in the temperature range from room temperature to 1100 ℃. Plotted versus the temperature, the electrical conductivity of the amorphous compacts exhibits a non-linear behaviour, which is related to the high density of states within the "mobility gap". In most of the studied temperature region, the investigated samples show semiconducting behaviour with a positive temperature coefficient of the electrical conductivity, but for sample(a) and (d),negative temperature coefficient region is also found. Both positive and negative temperature coefficient of conductivity have been discussed. The conductivity variation with temperature in different regions for all these samples shows an Arrhenius law dependence on 1/T, and is also influenced by phonon scatter to lead fluctuation of conductivity which corresponds to the negative temperature coefficient region. In general , there are three transport mechanisms in our investigated temperature region. The conductivity could be dramatically increased upon annealing at higher temperature, this is attribute to a change of solid state structure of these amorphous phase owing to the loss of residual hydrogen with increasing temperature. Small amount boron doping contributed...